1. Field of the Invention
The present invention relates to an image forming apparatus such as a facsimile machine, a printer, or a copier, and more particularly to an image forming apparatus including a postprocessing device, such as a stapler for stapling a bundle of sheets or a punching device for forming punched holes.
2. Description of the Related Art
An image forming apparatus including a postprocessing device, such as a stapler for stapling a bundle of sheets on which images have been recorded and thereafter discharged, and a punching unit for forming punched holes on a bundle of sheets, has hitherto been known. There has also been known another image forming apparatus, wherein, when a sheet is transported to a postprocessing device from an image forming apparatus main body where an image is recorded on the sheet, a postprocessing sheet transport device including a transport path for transporting the sheet to the postprocessing device is attached on a main body discharge tray of the image forming apparatus main body. JP-A-10-152241 and JP-A-9-301602 disclose an image forming apparatus including the postprocessing sheet transport device.
JP-A-10-152241 discloses a postprocessing sheet transport device which includes a transport mechanism (12) for transporting a sheet to a sheet postprocessing device, and a discharge member for discharging a sheet to an optional discharge tray (a stack section 21) formed on an upper surface of a transport device, and which is attached to the upper surface of the discharge tray (a stack section 9) of the image forming apparatus main body. According to the technique disclosed in JP-A-10-152241, inside the postprocessing sheet transport device a switch is made in the transport direction of a sheet discharged from a discharge roller (8) of the image forming apparatus main body, thereby transporting the sheet to the optional discharge tray or to the postprocessing device.
JP-A-9-301602 discloses a postprocessing sheet transport device (relay transport means 16, 120) which is identical with the postprocessing sheet transport device described in JP-A-10-152241. JP-A-9-301602 also describes a postprocessing sheet transport device, wherein an optional discharge device (a one-bin device 121) is disposed above a discharge roller of an image forming apparatus main body; and a discharge tray (125) on which a sheet discharged from the optional discharge device (121) is placed is disposed further above a discharge tray (transport cover 38) on the upper surface of the postprocessing sheet transport device.
However, in JP-A-10-152241 and JP-A-9-301602, the destination of a transport (the postprocessing device or the optional discharge tray) of the sheet which has been transported to the postprocessing sheet transport device by the discharge member (discharge roller) of the main body is switched inside the postprocessing transport device. Therefore, in JP-A-10-152241 and JP-A-9-301602, a switching device must be provided inside the postprocessing sheet transport device for switching a transport path between two transport paths. Accordingly, there arises a problem that a structure of the postprocessing sheet transport device is complicated, and cost is increased.
In addition, there arises another problem that, since the switching device or the like is disposed, the optional discharge tray in the postprocessing device transport device is shorter in length than a discharge tray of the main body, whereby a sheet that can be housed in the optional discharge tray is decreased in size. When the optional discharge tray is increased in length in order to solve the problem, there arises a problem that the postprocessing sheet transport device and the image forming apparatus are increased in overall size.
Meanwhile, when the discharge tray for the optional discharge device described in JP-A-9-301602 is disposed, a gap between the discharge tray of the postprocessing sheet transport device and the discharge tray of the optional discharge device is narrowed. Therefore, there arises a problem that the number of sheets that can be stacked on the discharge tray of the postprocessing sheet transport device is decreased. In addition, there arises another problem that a narrow gap between the trays makes it difficult for a user to take out a sheet from the discharge tray of the postprocessing transport device.